I am going to have the same worries, my wife didn't want me to use half of the 20 x 20 garage. She was worried about fire. A fire in that garage would be a home run,. She bought me an 8 x 12 shed kit. Bad enough I am on the downside of the hill of life, now I've go to build a shed! I have been giving this quite a bit of thought. Flooring will be used pressure treated decking, I all ready have it. Cement wall board above and around the forge. My chimney will be the Hofi style which will keep my roof water proof. Sheet metal, what ever scrap I can find to cover the floor. Most importantly a floor plan for my equipment that will allow a quick exit in case of emergency. Ventilation will be a couple of small windows.